1. Field of the Invention
The present invention relates to a sintered composite of silicon carbide and silicon nitride, used as a material for various structural parts in the fields of automobile, mechanical device, chemical device, aerospace device and the like, and a method of producing the sintered composite.
2. Description of the Prior Art
A sintered body containing silicon nitride as a main component thereof is chemically stable at room temperature and a higher temperature. This sintered body has a high mechanical is strength and thus is suitable as a material for bearing, turbocharger rotor and the like. Hitherto, high-strength silicon nitride sintered bodies have been prepared by a liquid-phase sintering at a temperature not lower than 1700.degree. C., using an .alpha.-silicon nitride powder and an oxide-type sintering aid. These sintered bodies have high strength and toughness at room temperature. However, the strength of these sintered bodies decreases at a high temperature not lower than 1,000.degree. C., because the added oxide-type sintering aid softens at this high temperature.
For the purpose of improving the above-mentioned silicon nitride sintered body in strength at high temperature, there have been provided the following first, second, third and fourth methods. In the first method, a normal oxide-type sintering aid is replaced by a high-melting-point sintering aid (see J. Am. Ceram. Soc., vol. 71, No. 3, pp. C-114 to C-147, 1988). In the second method, the amount of the oxide-type sintering aid is decreased (see Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi, vol. 97, No. 6, pp. 673-675, 1989). In the third method, a solid solution (sialon) is formed by dissolving the oxide-type sintering aid in silicon nitride grains, after the sintering (see J. Mater. Sci. vol. 14, pp. 2309-2316, 1979). In the fourth method, silicon carbide grains are dispersed at the boundary of and within silicon nitride grains (see Funtai Oyobi Funmatshu Yakin, vol. 39, No. 12, pp. 1119-1123, 1992). These methods are somewhat effective for improving the strength of the silicon nitride sintered body in strength at high temperature.
However, according to the above-mentioned methods, it is not possible to obtain a material which is high in both strength and toughness at high temperature. In fact, according to the first method, MgO as a sintering aid is replaced by Y.sub.2 O.sub.3. With this, the temperature at which the strength decreases is increased from 1,000.degree. C. (MgO) to 1,200.degree. C. (Y.sub.2 O.sub.3). However, this improvement in strength at high temperature is still insufficient. According to the second method, the amount of oxide-type sintering aid is decreased from 10 mol % to 1 mol %. With this, the temperature at which the strength decreases is increased from 1,000.degree. C. to 1,200.degree. C. However, this improvement is also insufficient. According to the third method, it is possible to obtain a material of which strength does not deteriorate up to 1,400.degree. C. However, this material has a low fracture toughness not higher than 5MPa.sqroot.m. A sintered body prepared by the fourth method is also improved in strength at high temperature, but not in toughness.
Thus, there has been an increasing demand for a ceramic composite which is high in strength and toughness at high temperature as well as at room temperature.